Microbore rad lukewarm-hot-cold

12 Oct 2009
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United Kingdom
My mum has microbore on a combi system. 1 rad was completely stone cold including the pipework to it. Have cleaned out the rad and closed the other radiators off but nothing. Tried the pin but it moves freely.

Today I took the rad off, opened the lockshield - nice and hot but when I opened the trv a little water then nothing. I've drained the system and taken the trv off to make sure of no blockages then filled it back up. The pipes got hot but the rad only got hot in the bottom corner, cranked open the return hot water through then lukewarm then totally cold again.

Could it be a faulty valve? If so is there a way that I can create a vaccum like on a conventional system so I don't have to drain down again. Am going to add inhibitor to the offending rad as well once I can get the damn thing working!

The boiler installation and last inhibitor injection was 18 months ago and it has worked.

Any hints welcome!
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With the radiator removed and making sure the system pressure is well up to 1 bar at least, open the TRV for a good squirt into a bucket.
Draw off a fair amount of water, (re-pressurising again if necessary) and see if any gunge comes out.

Do the same for the lockshield valve and this may sufficiently clean out the pipework to get the radiator going again.


When the TRV was taken off did you check it was opening by blowing through it?
i did this return pumped out hot water but trv spitted a bit out then stopped hence i drained it down, that got it working for a very little bit then nothing again.

I used clear pipe when i drained out and it was as clear as day so if its a blockage can't be anything but air but why would it start working then stop again?
When you repressurised the boiler and opened the TRV did anything at all come out? This is obviously the problem area but it isn't clear whether it's the valve or the pipework that's at fault.
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You could try partially loosening the pipe connection to the TRV to see if water starts to escape.

If you get around to removing the TRV again have a blow through it to see if it's opening OK.

probably blocked at the manifold
what does the manifold look like? I've seen it come up in other threads but have no idea what it is
have no idea. Am going to change the trv over with one thats definately working, if its still cold then am gonna have to get somebody in with more knowledge then me!

Thanks guys
When you get the new TRV in place and before replacing the radiator, if you still can't get much water to squirt out of it on test, try raising the system pressure up to 2 bar temporarily and see if that helps shift any blockage.

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